1. Field of the Invention
This invention relates generally to phased array antennas. More particularly, this invention relates to a phased array antenna incorporating a phase error correction system in which phase adjustments are made to correct for array structural deformation, and individual array element phase errors. Phase adjustments are also made to affect phase coherence of all lens elements in the array by a feedback loop at each element.
2. Background Art
Phased array antennas are composed of a group of individual radiators distributed and oriented in a one or two dimensional spacial configuration. The amplitude and phase excitations of each radiator is individually controlled to form a radiated beam of any desired shape in space, such as a pencil beam or fan beam. The position of the beam in space is then controlled electronically by adjusting the phase of the excitation signals at each of the individual radiators. Computers are utilized to control the digital beam steering function for the phased array antenna. As is well known in the prior art, in general, most of the feed systems for forming beams for space based phased array antennas can be categorized into two basic groups, namely optical space feeds, and constrained feeds.
In optical space feeds, illumination of the array elements in an optically space fed antenna is accomplished by optically distributing the source signal (transmitter) through space for illuminating an array of pick-up horns or elements which are connected to the radiating elements. Two basic types of optical space feeds are the transmission type and reflection type. A conventional transmission type space feed is shown in FIG. 1A. While the advantage of the optical space feed is its simplicity, its major disadvantage is an excessive volume of physical space required to accommodate the feed system. Space feeds require large extended feeds which are essential to reduce beam sidelobes and reduce spill-over signals, but which are awkward to deploy in space. Furthermore large extended feeds present problems in beam steerage due to the displacement of the extended feed from its true focus position, and the difficulties in maintaining it at that position.
Constrained feeds utilize feed transmission lines to each element of the array. This is accomplished typically by end fed serial feed arrangements in which the elements are arranged serially along a main line, or a corporate fed arrangement where each lens element is individually fed from the energy source. A conventional corporate fed array is illustrated in FIG. 1B. In constrained feed systems the path length to each radiating element must be computed, and it is critical that the path length be correct to a very high degree. Unique to corporate or end fed arrays is the problem of maintaining of phase coherence to each element of the array through the feed transmission lines in the face of unequal exposure to the sun's radiations.
In addition, there are phase stability problems faced by any type of phased array antenna, whether space fed, corporate fed, end fed or otherwise. Phase stability problems can result from a change in array shape due to structural deformation and flexibility of the structures. These structural changes affect the signal path lengths, and thus cause unwanted shifts in signal phase resulting in phase errors. In addition to structural changes, there are varying phase shifts which are functions of temperature and aging of components. Such deformations and phase errors, if uncorrected, cause aperture phase errors which result in undesirable side lobes in the antenna beam.
Regarding space based radar systems utilizing phased array antennas, prior art systems are designed to minimize the large cost of placing a payload into orbit. Therefore, the antenna structures are typically designed to be as light in weight as possible. Unfortunately, such lightweight construction usually results in flexible antenna structures which are more prone to suffer deformations due to exposure to unequal solar radiations, heating, pressure, and orbital instabilities. Thus, the problems associated with phased array antennas for use in space based radar systems become even more acute.
The present invention recognizes the need for reducing feed tolerance problems of both space fed and corporate fed phased array lenses. In addition, the present invention automatically corrects for any remaining lens structural deformations in the phased array antenna, and compensates both for phase changes caused by component aging, and for phase changes caused by temperature changes or inequalities.
In light of the above, it therefore is an object of the present invention to provide a phase error correcting system which provides corrections in phase error due to any structural deformation of the phased array antenna.
It is a further object of the present invention to provide a phase error correcting system which compensates for phase changes caused by component aging, temperature changes or variations, or any other causes of phase error.
It is yet another object of the present invention to reduce the feed tolerance problems encountered in both space fed and corporate fed phased array lenses.
A further object of the present invention is to eliminate the extended feed and its large feed boom associated with space fed phased array lenses.
It is yet another object of the present invention to correct within a few electrical degrees for path length differences which may occur in corporate fed arrays.
It is an object of the present invention to compensate for all types of phase errors from which spaced based radar lens phased arrays suffer.
Yet another object of the present invention is to achieve array phase coherence on both transmit and receive pulses without using conventional space fed or corporate feeds.
It is yet a further object of the present invention to provide a phase error correcting system which is relatively easy to manufacture and which is cost effective in use.